Although several plant workers are ill from radioactive exposure in Japan, the radiation risk to the public appears low so far, experts said.

“At least as of now, what we’re looking at is rather more like Three Mile Island than Chernobyl,” said Dr. David J. Brenner, director of the Center for Radiological Research at Columbia University.

The radiation release from the 1986 Chernobyl disaster, where the entire reactor blew up and vaporized its radioactive fuel, was about a million times the amount released from the partial core meltdown at Three Mile Island in 1979, he said. The Chernobyl accident led to an epidemic of thyroid cancer and increases in leukemia, he said.

But from Three Mile Island, Dr. Brenner said, “There is no evidence that anybody at all got sick, even decades later.”

At the exposure rate now being reported at the boundary of the Fukushima Daiichi plant, it would take many weeks before people exposed would notice any symptoms.

High levels of exposure can cause severe radiation sickness and death. Symptoms can include nausea, fatigue, vomiting, hair loss, diarrhea and hemorrhaging.

Even high doses generally take several weeks to cause death.

“It’s normally due to what we call ‘gut death,’ ” Dr. Brenner said. “The lining of the gut gets depleted.”

Radiation interferes with the cells’ ability to divide and reproduce, and cells in the intestine are usually replaced frequently. For the same reason, the blood-forming cells in bone marrow are also sensitive to radiation. “What you really die of in the end is infection,” Dr. Brenner said.

The more likely risk for the public is that of low-level exposures, which can increase the risk of cancer many years later. Again, the danger depends on the length of exposure and what types of radioactive materials to which one is exposed.

Some radioactive materials are readily absorbed by the body and linger there. Iodine, for example, goes to the thyroid gland, and strontium to bone, and they emit radiation inside the body that over time can lead to cancer or leukemia. Other radioactive materials, like tritium, pass quickly through the body.

The Japanese government is handing out iodine pills to flood the thyroid gland with ordinary iodine in hopes of preventing it from taking up the radioactive form.

Dr. Brenner said the iodine pills were protective, but were “a bit of a myth” because their use is based on the belief that the risk is from inhaling radioactive iodine. Actually, he said, 98 percent of people’s exposure comes from milk and other dairy products.

“The way radioactive iodine gets into human beings is an indirect route,” he said. “It falls to the ground, cows eat it and make milk with radioactive iodine, and you get it from drinking the milk. You get very little from inhaling it. The way to prevent it is just to stop people from drinking the milk.” He said that the epidemic of thyroid cancer around Chernobyl could have been prevented if the government had immediately stopped people from drinking milk.

Crops can also be contaminated. “I wouldn’t be eating an apple from a tree close to the plant,” Dr. Brenner said.

Children, and fetuses, are more vulnerable to radiation than are adults. Scientists estimate that about 5 percent of the population is genetically more susceptible to radiation than the rest.

The radioactive elements released from the reactor form clouds that are carried off by the prevailing winds. Again, the risk depends on how much is released. “As it’s being blown away, to some extent it’s being dispersed,” Dr. Brenner said. “And some of it falls on the ground.”

One way of measuring radiation exposure is in a unit called the rem. According to the Environmental Protection Agency, most people in the United States receive 0.3 rem per year just from normal, background radiation. Flying for 12 hours at 39,000 feet exposes a person to 0.006 rem. At 5 to 10 rem, lab tests can pick up changes in blood chemistry. Nausea starts after 50 rem, hemorrhaging at 100 rem. At 500 rem, half of people exposed will die within 30 days. At 2,000 rem, a person can die within hours or days. So far, one employee at a nuclear plant in Japan has been reported to have had an exposure of 10 rem, not enough to produce obvious symptoms. The annual dose limit for workers at nuclear plants in the United States is 5 rem.

People are so afraid of radiation that any threat of exposure can cause what Dr. Brenner called psychophysical consequences. He cited an incident in 1987 in Goiania, Brazil, in which people were exposed to radioactive material stolen from a hospital. Fearing contamination, about 125,000 sought medical exams. Thousands reported symptoms of radiation sickness, like vomiting and rashes. Ultimately, only 249 turned out to have any signs of contamination.

A version of this article appears in print on March 14, 2011, on page A9 of the New York edition with the headline: Several Plant Workers Are Ill, but Radiation Risk in Japan Is Seen as Low for Now. Order Reprints|Today's Paper|Subscribe